The invention is directed primarily at those aerofoils which form part of non-rotating stators, but by suitable adaptation may be utilised in the rotary aerofoils of turbine blades.
It is known to cool turbine stators by forming them from hollow structures and passing pressurised air through them. In order to ensure maximum cooling of the inner surface of the aerofoils of the stators, passageways are formed by joining opposing surfaces with walls which lie along the length of the aerofoil interior, and the cooling air caused to flow along those passageways.
The elongate walls act as fins which extract heat from the aerofoil by conduction and pass it to the air which flows in contact therewith. Clearly, in order that the air may extract the heat efficiently, it must flow in contact with the walls. Further, the air pressure must also be maintained as high as possible, despite the normally tortuous path it is constrained to follow through the aerofoil.
Whilst the air is confined by the walls, both criteria are met, however, when the air again reverses its direction of flow over an end of the last wall in cooling air flow series, it tends to break away from the downstream surface of that wall, thus reducing its ability to remove heat from the downstream side of the last wall.
British patent GB2165315B achieves delaying of breakaway by providing a combined deflector/entrainer in the form of a curved member, aided by an aerodynamically shaped wall end. The constraining effect of the member and the shape of the wall end ensures that air passing therebetween does so such that on leaving that area, it tends to stay attached to the downstream surface of the wall. Further, the outer surface of the curved member is so shaped as to provide a Coanda effect on the air flowing thereover and that airflow consequently leaves the member in a direction parallel with the ejected airflow, rather than meeting it in a manner which would cause turbulence and breakaway.
The benefit, though real relative to art prior to GB2165315B, is short lived, because breakaway does occur further along the length of the downstream side of the wall, with consequent loss of cooling efficiency.